Hard Disk Drives (HDDs) spin magnetic disks and drive head gimbal assemblies (HGAs) at high speed in response to requests for larger capacity, higher recording density, and higher-speed accessing. As a result, some instability of air (turbulence) arises to buffet the magnetic disks and the HGAs.
This turbulence buffeting can become a big disturbance in positioning heads for data recorded on a magnetic disk with high density. This is because that the turbulence occurs at random and it is difficult to estimate its magnitude and cycle so that swift and accurate positioning control becomes complex and difficult. The turbulence buffeting may cause noise and impair the quietness of the device.
Another problem caused by influence of the air inside the device due to the high speed spin of the disk, is increased electric power consumption. When the magnetic disk is spun at a high speed, the air around the disk is drawn and spun together. In contrast, the air away from the magnetic disk remains still, so that shearing force arises between them to become a load against the spin of the disk. This is called windage loss, which becomes greater as the disk spins at a higher speed. To spin the magnetic disk at high speed against this windage loss, a motor will require a greater power output and more electric power.
To suppress the flutter vibration of a magnetic disk caused by turbulence, an idea has been proposed to disposes a shroud in the vicinity of the circumferential edge of the magnetic disk (for example, refer to Japanese Patent Publication No. 2000-322870 “Patent Document 1”). A smaller gap between the shroud and the outer periphery of the magnetic disk reduces turbulence of the swirling flow caused by spins of the magnetic disk to reduce the flutter vibration of the magnetic disk.
In addition, focusing on the fact that the air turbulence and the windage loss are proportional to the density of the gas inside the device, an idea has been proposed to enclose a low density gas instead of air in a hermetically sealed HDD to reduce the air turbulence and the windage loss. Hydrogen, helium, and the like may be examples of the low density gas, but helium is optimum since it is effective, stable, and safe in considering the actual use. An HDD with helium gas sealed therein can overcome the above problems and achieve swift and accurate positioning control, power saving, and satisfactory quietness.
However, helium has very small molecules and a large diffusion coefficient. Therefore, there has been a problem that an enclosure used in a common HDD is sealed so poorly that the helium gas leaks out easily. In order to make it possible to seal in low density gas such as helium gas, a technique in the Japanese Patent Publication No. 2007-280555 (“Patent Document 2”) has been proposed.
To suppress the flutter vibration of the magnetic disk caused by turbulence in an HDD with helium gas sealed therein, it is preferable to reduce the gap between the circumferential edge of the magnetic disk and the wall facing the circumferential edge as much as possible. If a shroud is provided to suppress the flutter vibration of the magnetic disk, it is necessary that the shroud's side which faces the magnetic disk be positioned with high precision to reduce the gap between the shroud and the magnetic disk.
However, as in the technique disclosed in the Patent Document 1, if a shroud is provided as a component different from a base, it is difficult to dispose the shroud within the base with high positioning precision. Since the track pitch of current magnetic disks is very small, if there is such an error in positioning of the shroud, it is difficult to suppress the flutter vibration of the magnetic disks to be small enough for highly accurate head positioning.
To obtain a high precision in positioning the shroud, it is preferable to form the shroud integrally with the base. For example, the exterior wall of the base can be used as a shroud. Forming the exterior wall of the base thicker and the inner surface thereof to be curved around the circumferential edge of the magnetic disk achieves a smaller gap between the circumferential edge of the magnetic disk and the inner surface of the exterior wall.
Typically, the base is made by die-casting with aluminum alloy. Die-casting, however, does not exhibit very high processing accuracy. Besides, a draft angle is required to take a casting out of a mold. Thus, if a plurality of magnetic disks are present, the gap between the inner surface of the exterior wall and the circumferential edge of each magnetic disk is different in size depending on the position of the magnetic disk.
Accordingly, to attain a desired small amount of the gap between the circumferential edge of the magnetic disk and the inner surface of the exterior wall, it is necessary to cut and shape the surface facing the magnetic disk. In the meantime, there is a problem in the die-casting: formation of shrinkage cavities caused by shrinkage. The shrinkage cavities are tiny cavities formed inside the base formed by die-casting and causes leakage of low density gas. The shrinkage becomes larger as the wall becomes thicker and less uniform, so that the shrinkage cavities are likely be formed.
Shrinkage cavities are formed inside the base manufactured by die-casting, but not on the surface of the base. The dense layer of the surface is called a skin layer. The low density gas cannot pass through the skin layer so that the skin layer can prevent leakage of the low density gas. However, as in the above-description, when the inner surface of the base's exterior wall is cut, the skin layer is removed so that the inner layer with shrinkage cavities therein is exposed. This increases the possibility of leakage of the low density gas in the enclosure through the exterior wall to the outside.